Liquefied natural gas (LNG) is obtained by liquefying natural gas, one of the fossil fuels, and LNG storage tanks may be divided into onshore storage tanks installed on the ground or buried in the ground or mobile storage tanks installed in vehicles such as automobiles and ships, depending on installation location.
Since LNG has a danger of explosion when exposed to impact and is kept in a cryogenic state, a storage tank for LNG has a structure in which impact resistance and liquid tightness are maintained firmly.
In addition, in contrast to onshore storage tanks where sloshing of LNG is negligible, LNG storage tanks installed in automobiles and ships should be resistant to mechanical stress due to the sloshing. However, since LNG storage tanks installed on ships equipped with measures against mechanical stress can also be used as onshore storage tanks, the structure of an LNG storage tank installed on a ship will be described as an example herein.
FIG. 1 is a schematic sectional view of a ship equipped with a typical LNG storage tank.
Referring to FIG. 1, a ship equipped with an LNG storage tank generally has a double-hull structure including an outer wall 16 forming an outer shape and an inner wall 12 formed inside the outer wall 16. The inner wall 12 and the outer wall 16 of the ship 1 may be connected to each other through a reinforcing member such as a connecting rib so as to be integrally formed with each other. Alternatively, the ship may have a single-hull structure without the inner wall 12.
In addition, the interior of the hull, that is, the interior of the inner wall 12, may be divided by one or more bulkheads 14. The bulkhead 14 may be formed of a cofferdam installed in a conventional LNG carrier 1, as known in the art.
Each of the internal spaces divided by the bulkhead 14 may be utilized as a storage tank 10 for storing cryogenic liquid such as LNG.
Here, an inner wall surface of the storage tank 10 is sealed in a liquid-tight manner by a sealing wall. That is, the sealing wall has a structure in which plural metal plates are integrally connected to one another by welding to form a single storage space, whereby the storage tank 10 can store and transport LNG without leakage.
The sealing wall directly contacting ultra-low temperature LNG may have a corrugated portion to cope with temperature change caused by loading/unloading of LNG.
The sealing wall 50 is securely connected to the inner wall 12 or the bulkhead 14 of the ship 1 through a plurality of anchor structures 30. Thus, the sealing wall 50 cannot be moved relative to the hull.
A heat insulating wall is interposed between the sealing wall 50 and the inner wall 12 or the bulkhead 14 to form a heat insulating layer. The heat insulating wall is composed of a corner structure 20 disposed at a corner of the storage tank 10, an anchor structure (not shown) disposed around an anchor member, and a planar structure 40 disposed on a flat portion of the storage tank 10. In other words, the corner structure 100, the anchor structure, and the planar structure 200 are arranged to form a whole heat insulating layer in the storage tank 10. That is, the insulating layer may be entirely provided to the storage tank 10 by the corner structure 20, the anchor structure 30, and the planar structure 40.
Here, the anchor structure 30 includes a rod-shaped anchor member directly connecting and securing the sealing wall to the hull and an insulator disposed around the anchor member.
In addition, the sealing wall 50 is mainly supported by the anchor structure 30, and the corner structure 20 and the planar structure 470 only support the load of LNG applied to the sealing wall 50 and are not directly coupled to the anchor structure 30.
FIG. 2 is a sectional view of a portion of a typical LNG storage tank, as disclosed in Korean Patent No. 499,710.
Referring to FIG. 2, the typical LNG storage tank 10 includes secondary heat insulating walls 22, 32, 42 and primary heat insulating walls 24, 34, 44 sequentially formed on an inner wall 12 or a bulkhead 14 constituting a part of a hull; and secondary sealing walls 23, 33, 43 interposed between the secondary heat insulating walls 22, 32, 42 and the primary heat insulating walls 24, 34, 44. In addition, a primary sealing wall 50 is disposed on the primary insulating walls 24, 34, 44.
The LNG storage tank 10 includes corner structures 20 disposed at inner corners thereof, anchor structures 30 disposed at certain intervals on a bottom surface thereof, and planar structures 40 interposed between the corner structures 20 or the anchor structures 30 to be slidable. Here, the corner structure 20, the anchor structure 30, and the planar structure 40 are preliminarily manufactured as unit modules and then assembled to the storage tank 10. Then, the primary sealing wall 50 is disposed thereon to seal the heat insulating walls in a liquid-tight manner, thereby providing a space for storing LNG inside the primary sealing wall.
As shown in FIG. 2, the corner structure 20, the anchor structure 30, and the planar structure 40 include the primary heat insulating walls 24, 34, 44, the secondary heat insulating walls 22, 32, 42 and the secondary sealing walls 23, 43, respectively, and are collectively referred to as heat insulating wall structures 20, 30, 40.
In the heat insulating wall structures 20, 30, 40, the secondary sealing wall and heat insulating walls of each of the unit modules are bonded together via adhesives so as to be integrally formed with one another. Generally, the secondary heat insulating walls 22, 32, 42 are composed of polyurethane foam, which is an insulating material, and a plate attached under the polyurethane foam. The primary heat insulating walls 24, 34, 44 are composed of polyurethane foam and a plate adhered thereto with adhesives. Further, the primary sealing wall is disposed on the primary heat insulating walls 24, 34, 44 and is welded to the anchor structure 30.
In addition, the secondary heat insulating wall 42 of the planar structure 40 is formed at a lower end thereof with a flange 42a, which is larger than the secondary heat insulating wall 42. The flange 42a is inserted into a groove formed in a lower end of the anchor structure 30 to be slightly slidable.
In the illustrated example, each of the anchor structures 30 includes an anchor support rod 36, a lower securing member 37, a secondary anchor insulation wall 32 and a primary anchor insulation wall 34, and a secondary sealing wall 33 is interposed between the secondary anchor insulation wall 32 and the primary anchor insulation wall 34. The anchor support rod 36 is connected at one end thereof to the primary sealing wall 50, and is connected at the other end thereof to the inner wall 420 of the hull through the securing member 37.
The primary sealing wall 50 is welded to an upper end of the anchor support rod 36 of the anchor structure 30.
The anchor structure 30 is placed at the connection point between the neighboring planar structures 40 to interconnect the planar structures, and the planar structure 40 is secured to the inner wall 12 of the hull or the bulkhead 14, constituting the storage tank 10. In addition, the securing member 37 of the anchor structure 30 is disposed around the anchor support rod 36.
In the typical LNG storage tank, the heat insulating wall structure is composed of the primary and secondary heat insulating walls and the primary and secondary sealing walls and thus has a complicated structure. In addition, the structure for connecting the secondary sealing wall of the LNG storage tank is complicated and installation of the heat insulating wall is not easy. Further, there is a possibility that sealing reliability of the secondary sealing wall is reduced, causing LNG leakage, since the structure of a connection of the anchor structure or the secondary sealing wall is complicated and installation of the anchor structure or the secondary sealing wall is difficult.
In addition, the typical corner structure 20, which only supports the load of LNG applied to the sealing wall 50 without supporting the sealing wall 50, has a problem in absorption of stress due to thermal deformation of the storage tank caused by loading/unloading of LNG at extremely low temperature or due to deformation of the hull.
In order to solve these problems, storage tanks having a new structure different from that of typical LNG storage tanks have been proposed to reduce boil-off gas (BOG), which is a loss due to vaporization of LNG having a liquid phase at ultra-low temperature, while simplifying the structure of a storage tank and a manufacturing process. Therefore, there is a need for a corner structure having a new structure.